The present invention relates to the field of gear mechanisms, and in particular to a gear mechanism that includes a plurality of planetary gears.
Conventional gear mechanisms include a first gear stage set in a fixed housing and planetary wheels, which drives a second gear stage that meshes with a drive shaft. One example of such a two-stage gear mechanism is shown for example in German Patent Specification DE 43 24 876 C1. The gear mechanism disclosed therein consists of two planetary gear stages connected one after the other. The entire gear mechanism is designed as a rotary gear mechanism, in which the individual planetary wheels of the planetary gear stages execute a rotational motion about the center axis of the gear mechanism, such that the individual planetary wheels of the planetary gear stages themselves again rotate about their own axis.
Such known gear mechanisms have the problem that the angular momentum of the rotating planetary wheels can lead to friction losses and stability problems that reduce the efficiency of the gear mechanism. Furthermore, a structure consisting of two planetary gear stages connected one after the other requires a relatively large amount of space.
Therefore, there is a need for a gear mechanism that reduces the rotating moments of inertia and at the same time reduces the overall structural space required by the gear mechanism.
Briefly, according to an aspect of the present invention, a two-stage gear mechanism includes a drive shaft having a first gear stage disposed in a fixed housing and containing planetary wheels. The two-stage gear mechanism also includes a second gear stage, driven by the first gear stage and meshing with a drive shaft. Each of the planetary wheels is formed as a stepped double gear wheel with a first gear wheel that engages the drive shaft, and with a second gear wheel that points toward the direction of the output side. The bearing axles of each of the planetary wheels are disposed spatially fixed within the housing. The second gear wheels of the planetary wheels mesh with a hollow wheel of the second gear stage, and the hollow wheel is non-rotationally coupled to the drive shaft.
Each of the planetary wheels is formed as a stepped double gear wheel, with a first gear wheel that engages the drive shaft, and a second gear wheel, pointing toward the direction of the output side. The bearing axles of each of the planetary wheels are disposed spatially fixed within the housing. The second gear wheels of the planetary wheels mesh with a hollow wheel of the second gear stage, the hollow wheel being non-rotationally coupled to the drive shaft.
Significantly, the planetary wheels rotate about their own axis, but not about the center axis of the gear mechanism, as is the case with previously-known planetary gears. The first gear stage of the present invention has planetary wheels which are each designed as stationary stepped double gear wheels. It is used to step down the rotary motion of a pinion seated on a drive shaft. The pinion is coupled, for example, to an electric motor. In combination with the second gear stage, not only is the rotary motion stepped down but at the same time the torque at the drive shaft is increased. Due to the spatially fixed planetary wheels, the gear mechanism allows a reduction of the rotating moments of inertia.
The second gear stage is designed as a hollow wheel that is non-rotationally coupled to the drive shaft, and which is driven by the spatially fixed planetary wheels of the first gear stage. The hollow wheel is preferably mounted at two points, such as for example at the free end of the drive shaft, and within the gear housing at a second bearing that is seated on a gear casing fixed within the housing.
The space-saving structure of the inventive gear mechanism not only allows the torque to be taken off at the output end, but also the transverse force to be applied at the output shaft end. Thus, the gear mechanism is especially suited as an actuator for engine components in a motor vehicle, and especially in an exhaust gas recycling system of a motor vehicle.
Several (e.g., three) planetary wheels constructed as stepped double gear wheels can be driven simultaneously by the drive shaft. For example, the first gear stage can include a cylindrical tooth gearing with three spatially fixed planetary wheels. The rotary motion is symmetrically introduced through such a plurality of planetary wheels. In addition, the rotating masses are reduced relative to conventional planetary gear mechanisms by the spatially fixed arrangement of the planetary wheels, which rotate about their own axis. Reducing the rotating moments of inertia improves the efficiency of the overall gear mechanism and reduces friction losses.
A casing is preferably fixedly situated within the housing, and includes openings to accept the spatially fixed bearing axles of the planetary wheels. One end of each of the spatially fixed bearing axles of the planetary wheels is mounted in casing openings, while the other end is seated in openings of the gear housing.
The gear housing is preferably formed of two housing shells. One of the housing shells serves as a motor flange, preferably for an electric motor that is coupled to the gear mechanism. The housing has a second housing shell, which is pot-shaped and on which is rigidly seated the first housing shell (e.g., the above-mentioned motor flange). The second housing shell has a central bore in which is situated one of the bearing bushes for the drive shaft. The other bearing bush for the drive shaft is situated centric to the gear axis and is held by the gear casing.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.